Optical signals in fiber-optic networks may be switched either electronically, by converting the signal to electronic format and back, or optically in the signal""s native format. Electronic switching of optical networks can create bottlenecks caused by the time required to change the signal from optical to electronic format and back. Furthermore, the electronic switch element must be replaced whenever a network is upgraded to higher bandwidths or additional channels. In contrast, optical switches do not require signal conversion and are transparent to the number of channels, wavelength, or bandwidth.
Several technologies have been proposed for optical switching. The development of micro-electromechanical (MEMS) technology for fabricating large numbers of mirrors in small volumes has made it practical to use mirrors for switching optical signals in free space. There are two general configurations for mirror-based switching. In the first configuration, referred to as 2D or N2 designs, a mirror is associated with every possible pair of input and output fibers. The mirrors for this configuration have two distinct states, xe2x80x9conxe2x80x9d to connect the pair and xe2x80x9coffxe2x80x9d to disconnect them. The number of mirrors required in the switch increases as the square of the number of channels. The other configuration is the 3D or 2N configuration, in which each input and output fiber has a mirror associated with it, and the mirrors rotate to N different positions to connect the desired pairs. The number of mirrors increases linearly with the number of channels. Such a geometry is described, for example, by Callaway et al. xe2x80x9cArray Light Valve Switches Information Signals Between Fiber Optic Signal Conductors,xe2x80x9d IBM Technical Disclosure Bulletin, 27(2) 1984 pp. 1119-1120.
In the past, optical switches have tended to have high insertion losses, requiring the incorporation of optical amplifiers into the network. Optical switches also have strict alignment requirements. In existing implementations, optical mirror arrays used for switching have been mechanically separate from the optical fibers used to carry the signal and from other passive optical elements, requiring continuous monitoring of and adjustments to the alignment of the various system components. Most of the mirror switching fabrics proposed for optical network switching have been based on surface micromachined polysilicon technology, which limits the optical performance of the mirrors. Finally, optical switches typically have no mechanism for reading the data stream, and thus must be integrated with electronic logic circuits to read and decode the data and determine the required paths.
Lucent Technologies has disclosed an all optical network switch that uses biaxial MEMS mirrors to switch between as many as 256 input and output fibers. The mirrors are electrostatically actuated and gimbal mounted to provide the two dimensions of motion. The mirrors are arranged in a two-dimensional array, as are the optical fibers. The mirror and fiber arrays are aligned to face each other on an axis perpendicular to the fiber axis and the mirror normal. The optical system also incorporates a fixed mirror at an angle to the mirror array. Each fiber is associated with a mirror in the array; to connect an input fiber and an output fiber, the mirrors move to view each other through the fixed mirror. The mirrors, however, are surface micromachined of polysilicon, which limits both the surface quality of the mirror and the stiffness of the mirror body, limiting their optical performance. In this switch design, the MEMS chip that carries the mirror is packaged separately from the optical fibers and lenses and from the fixed mirror, which requires fine alignment both during the assembly and in use.
Optical Micro Machines (OMM) has demonstrated the switching of live network traffic using its optical switch based on polysilicon mirrors fabricated using standard CMOS and VLSI technology. The mirrors are gimballed to move in two dimensions. The current implementation uses so-called 2D technology.
Integrated Micromachines (IMMI) fabricates optical switches using mirrors fabricated of single crystal silicon using bulk, micromachining. This improves the optical quality of the mirrors and reduces the insertion loss of the switch to 1.5 dB. The mirrors are larger than used in competitive switches. This simplifies the optical alignment of the device, but results in a larger device size and increases the force that must be generated by the actuation mechanism. IMMI uses an electromagnetic drive for the mirrors, which can generate large forces but concomitantly uses more power.
The MEMS literature describes several other mirrors intended for use as optical switches. Most have not been demonstrated for switching, more than 2xc3x972 fibers. Toshiyoshi describes a silicon torsion mirror for use as a fiber optic switch. H. Toshiyoshi and H. Fujita, xe2x80x9cElectrostatic micro torsion mirrors for an optical switch matrixxe2x80x9d, J. Microelectromechanical Systems, 5(4) 1996. pp. 231-237. The Toshiyoshi mirror is a relatively large device (400 xcexcm on a side and 30 xcexcm thick) that rotates about an axis close to one edge of the mirror. The mirror is defined by etching the silicon wafer from the front, and the excess wafer material is etched from the back of the wafer. The supports are very thin, and the resonant frequency of the device is 75 Hz, too slow for network applications. The optical design is suitable for a 2D switch layout, and apparently has not been generalized into a commercially producible switch. A similar approach to switching is described by Dautartas et al.; see M. F. Dautartas et al., xe2x80x9cSilicon Based Moving Mirror Optical Switch,xe2x80x9d J. Lightwave Tech. 10(8) 1992, pp. 1078-1085.
Optical switch designs for switching between several input fibers and several output fibers exist in the patent literature. Young et al., in U.S. Pat. No. 6,091,967, have disclosed a design for an Mxc3x97N optical switch that uses, orthogonally aligned input and output fiber arrays with mirrors at the intersection of the optical path. This is a 2D design that uses a large number of mirrors for a moderate numbers of fibers. Furthermore, the details of the mirror fabrication, assembly, and alignment are not discussed. Bishop, in U.S. Pat. No. 6,031,946, has disclosed a switch consisting of two optical fibers with collimating and focusing optics and a moving mirror to switch between the on and off states. These patents generally do not discuss the detail of manufacturing the mirrors or of integrating the elements into a network switch device.
The creation of integrated optical systems on multiple wafers to be joined by bonding wafers and then dicing the wafer assembly into individual die has been disclosed by Harden et al. See U.S. Pat. Nos. 6,096,155 and 5,771,218. These patents teach methods for fabricating passive optical elements, and especially diffractive elements, on one or both sides of two wafers, by methods such as etching or embossing. In addition, they teach the attachment of free optical elements in a self-aligning fashion by using solder pads on both the wafer and the element. The two wafers are bonded using solder or adhesive, and the resulting stack is then diced and packaged. The bonding process protects the optical elements from the dicing slurry.
The invention described here is an optical switch based on MEMS (micro-electro-mechanical) mirrors for switching fiber-optic data networks. The MEMS chip that carries the switching mirror arrays also has alignment features and passive optical components. It is mated to a cover that incorporates corresponding alignment features and passive optical components and that also forms part of the package. The alignment features serve to ensure the correct position and orientation of the optical components in the package cover relative to the optical components on the silicon chip. The connection to the rest of the optical network is achieved via optical fibers which are held in position by being placed in grooves, also micromachined into the MEMS chip. The alignment elements and grooves serve to simplify the assembly of the device and to maintain the optical alignment in use. The mirrors are bulk micromachined, preserving the optical qualities of the semiconductor wafer. Additionally, detectors may be integrated into the MEMS chip or even into the mirrors themselves to interface with the logic circuits controlling the switch.
By incorporating the alignment elements and optics into the package, the present invention achieves a number of improvements over the prior art. The device is considerably smaller than can be achieved by aligning separately packaged, fiber arrays, mirrors, and optics. The alignment is easier to establish as well as more robust, reducing initial costs and allowing the device to be used in environments subject to mechanical disturbances, such as vibrations or high acceleration.
The switching mirrors are electrostatically actuated, with high switching bandwidths, very low power consumption, and high reflectivity. The other optical elements are passive and coated with high efficiency reflective coatings. Thus the resulting device is low in cost while permitting high optical bandwidth, rapid switching, and low insertion losses. The minimum size of the mirrors is determined by the maximum beam diameter, as determined by the optical system design.